In the normal human heart, illustrated in FIG. 1, deoxygenated blood flows into the right atrium through the superior vena cava and the inferior vena cava. The atrium contracts, allowing blood to flow into the right ventricle. When the ventricle contracts, the deoxygenated blood is pumped through the pulmonary artery to the lungs. Oxygenated blood returning from the lungs enters the left atrium. From the left atrium, the oxygenated blood flows into the left ventricle, which in turn pumps oxygenated blood to the body via the aorta and the lesser arteries.
This pumping action is repeated in a rhythmic cardiac cycle in which the ventricular chambers alternately contract and pump, then relax and fill. As best seen in FIG. 1, a series of one-way cardiac valves prevent backflow of the blood as it moves through the heart and the circulatory system. Between the atrial and ventricular chambers in the right and left sides of the heart are the tricuspid valve and the mitral valve, respectively. At the exits of the right and left ventricles are the pulmonic and aortic valves, respectively.
It is well known that various heart diseases may result in disorders of the cardiac valves. For example, diseases such as rheumatic fever can cause the shrinking or pulling apart of the valve orifice, while other diseases may result in endocarditis, an inflammation of the endocardium or lining membrane of the heart. The resulting defects in the valves hinder the normal functioning of the atrioventricular orifices and operation of the heart. More specifically, defects such as the narrowing of the valve opening, referred to as valvular stenosis, the defective closing of the valve, referred to as valvular insufficiency, result in an accumulation of blood in a heart cavity or regurgitation of blood past the valve. If uncorrected, prolonged valvular stenosis or insufficiency can cause damage to the heart muscle, which may eventually necessitate total valve replacement.
These defects may be associated with any of the cardiac valves, although they occur most commonly in the left heart. For example, if the aortic valve between the left ventricle and the aorta narrows, blood will accumulate in the left ventricle. Similarly, in the case of aortic valve insufficiency, the aortic valve does not close completely, and blood in the aorta flows back past the closed aortic valve and into the left ventricle when the ventricle relaxes.
In many cases, complete valve replacement is required. Mechanical artificial heart valves for humans are frequently fabricated from titanium, pyrolitic carbon or biologic tissue, including tissue from cows, pigs or humans. The more successful artificial heart valves are notable for their nonthrombogenic properties, i.e. their relatively low tendency to cause blood clots. Moreover, they are lightweight, hard and quite strong. Therefore, such valves have become widely accepted and used by many surgeons.
Mechanical prosthetic heart valves typically comprise a rigid orifice supporting one, two or three rigid occluders, or leaflets. The occluders pivot between open and shut positions and thereby control the flow of blood through the valve. The orifice and occluders are commonly formed of pyrolytic carbon, which is a particularly hard and wear resistant form of carbon. Because pyrolytic carbon is relatively brittle, the orifice is often surrounded by a stiffening ring, which may be made of titanium, cobalt chromium, or stainless steel. In one preferred valve configuration, the orifice and stiffening ring are captured within a knit fabric sewing or suture cuff. This prosthetic valve is placed into the valve opening and the sewing cuff is sutured to the patient's tissue. Over time, tissue grows into the fabric of the cuff, providing a secure seal for the prosthetic valve.
It has been found that the efficiency of a prosthetic heart valve is most dependent on the size of the valve opening. In other words, improved characteristics can be expected if the opening of the heart valve is made as large as possible with respect to the patient's anatomy. To accomplish this goal, the valve assembly should be made as radially thin as possible. In the past, some heart valves have been made with three metallic components that surround the orifice and leaflets: a central stiffening ring and upper and lower capture rings to capture the knit fabric tube of the sewing cuff. To hold the upper and lower rings in position, the stiffening ring has frequently been formed with grooves around its inside diameter, which serve to retain the capture rings against an outer side of heart valve annular body. This increases the radial bulk of the valve, however.
Furthermore, in many patients, once degeneration of a valve has occurred, it may occur that surrounding blood vessels are also diseased. Particularly in the case of the aortic valve, surgeons have found that the portion of the aorta adjacent to the valve is often degenerated to the degree that it must be replaced. Consequently, both the aortic valve and a segment of the ascending aorta may be replaced at the same time. This concept is illustrated in FIG. 2 and is described in detail in U.S. Pat. No. 5,123,919, which is hereby incorporated by reference. When this technique was being developed, the surgeon would stitch a segment of vascular graft to the sewing ring of the mechanical valve after implanting the mechanical heart valve. The juncture between the valve and the graft was abrupt and there was usually a sharp change of diameter to be expected between the valve and the graft.
Subsequently, a valve having a preattached graft was developed. The graft is typically attached inside the sewing ring. A major drawback of this configuration is that the valve size has to be reduced in order to accommodate the additional bulk of the graft end. Hence, the valve implanted with this combination is generally smaller than that which a surgeon would ordinarily implant. For example, a surgeon might be forced to implant a 25 mm valve when using an aortic valve/graft combination, when the tissue opening would otherwise suggest using a 27 mm valve. This results in a restriction in the available flow area, with associated resistance to flow. Furthermore, the orifice area (pressure drop across the valve) is proportional to the fourth order power of the internal diameter of the valve. Hence even the slightest diminution of the internal diameter is highly undesirable in the context of heart valve, as it greatly reduces the volume of blood that can be pumped with the available heart muscle.
With the foregoing in mind, it is an objective of the invention to provide a combined mechanical heart valve and graft which has an expanded valve orifice, corresponding to the diameter of the associated graft and tissue opening.
Another object of the invention is to produce a combined heart valve and attached graft wherein the graft is attached to the heart valve adjacent to the end of the sewing cuff so as to minimize the radial thickness of the combination.